Currently, devices using various kinds of media, such as optical disks, magnetic disks, magneto-optical disks, or the like, have been known in the art as data storage devices. In particular, hard disk drives (HDDs) have been widely used as storage devices of computers. Moreover, the HDDs have found application to moving image recording/reproducing apparatuses, car navigation systems, digital cameras, cellular phones, and the like in addition to conventional laptop and desktop computers.
An HDD comprises a head slider for accessing (reading or writing) a magnetic disk and an actuator for supporting the head slider and swinging to move the head slider above the magnetic disk. The actuator comprises a suspension on which the head slider is fixed. The airflow viscosity between the head slider and the spinning magnetic disk balances the force applied to the head slider by the suspension to allow the head slider to fly over the magnetic disk.
Solder ball bonding (SBB) has been known in the art as a method for electrically joining a slider and a wiring on a suspension (actuator). A conventional SBB method disposes solder balls between connection terminals of a slider and those of a transmission wiring disposed on the suspension and reflows them by laser beam to electrically interconnect the connection terminals of the slider and the transmission wiring on the suspension (for example, refer to Japanese Patent Publication No. 2007-128574 (“Patent Document 1”). The other end of the transmission wiring is connected to a substrate on which a preamplifier IC to be fixed to the actuator is mounted with solder. Recent request for lead-free materials demands that the above-described solder balls and a solder joint to be used in connecting the transmission wiring and the substrate be made of lead-free solder.
Diversification of applications for HDDs leads to diversification of the use environment for HDDs. For example, HDDs have recently come to be used under conditions where temperature changes drastically, like in car navigation systems. Specifically, an HDD is requested to be operable under severe conditions such as at a temperature of 90° C. (363 K) or −40° C. (233 K). Changes in environmental temperature cause application of a large thermal stress to a solder joint. In contrast, solder in common use has low material yield strength. Therefore, sufficient joining reliability in solder joining of a transmission wiring, particularly solder joining between a transmission wiring and a head slider, is demanded for thermal stress accompanying changes in temperature environment in use of an HDD.
A common material of lead-free solder is an alloy mainly composed of tin (Sn). The alloy undergoes phase transformation (transition) as described in “The Third Face of Tin”, Seizo Nagasaki, Kotai Butsuri, Solid Physics 1, (1967), pp. 47-51 (“Non-Patent Document 1”). That is, if the operation temperature is lowered to −40° C. (233 K), a phase transformation from the β-Sn structure to the α-Sn structure occurs so that the solder joint gets likely to break. In addition, “The Simple Hexagonal to β-Sn Martensitic Transformation in Sn-(7.0-9.5) at. % In Alloys”, Y. Koyama and H. Suzuki, Acta Metal. 37 (1989) pp 597-602 (“Non-Patent Document 2”) has reported that Sn-(7.0-9.5) at. % In alloys of the tin and indium (In) alloy system undergo martensitic transformation. If a martensitic transformation occurs between the simple hexagonal structure and the β-Sn structure, lenticular surface reliefs induced by the martensite are generated, which may develop a crack due to stress concentration caused by peaks and valleys of the reliefs.
Japanese Patent Publication No. 2007-141948 (“Patent Document 2”) discloses use of a Sn—In series alloy in solder joining between a semiconductor element mounting substrate and a heat sink. Further, it discloses a preferable solder composition of Sn—In series alloys containing no less than 14.5% of In. However, the Sn—In series alloys in this range include compositions which cannot attain mechanical reliability in a broad temperature range. Specifically, martensitic transformations accompanying changes in temperature, deformations at grain boundaries accompanying changes in solid solubility limit of the β phase, transformations of α-Sn at low temperature, or the like are not taken into consideration.
In Japanese Patent Publication No. 2007-105750 (“Patent Document 3”), a 1000-cycle test is disclosed with cycle condition of −40° C. to 125° C., 30 minutes of retention time, and 5 minutes of cycle transformation time. However, it does not propose the optimum range of a tin alloy in consideration of an aging effect caused by exposure at low temperature for a long time. Consequently, a solder joint is demanded which is highly reliable in actual use of an HDD at −40° C. to 90° C., and does not break even after exposure at low temperature for a long time.